1. Field of the Invention
The present invention relates to a carbon nanotube manufacturing apparatus, a carbon nanotube manufacturing method, and a radical producing apparatus.
2. Description of the Related Art
Carbon nanotubes have been attracting attention as a new material that has many superior characteristics derived from the special structure thereof. Examples of carbon nanotube manufacturing methods include arc discharge methods, laser ablation methods, and liquid phase methods. Of various carbon nanotube manufacturing methods, Chemical Vapor Deposition (CVD) methods are excellent in terms of productivity, controllability, and semiconductor process compatibility. Among CVD methods, for plasma CVD methods by which carbon nanotubes are manufactured by decomposing raw materials with plasma, a large number of manufacturing method examples that employ various plasma generating methods have been proposed. Examples of excitation methods include capacitive coupling plasma, inductive coupling plasma, and surface wave plasma.
For example, a technical document by N. Sakuma et al., in the Proceedings of the International Conference on New Diamond and Nano Carbons 2007, page 195, describes findings as a result of a study on growth of high-quality carbon nanotubes at a low temperature while a plasma CVD method is employed. According to the findings, to manufacture high-quality carbon nanotubes at a low temperature by using plasma, it is essential to selectively eliminate ion components because the ion components may cause etching in the carbon nanotubes.
It is possible to reduce the ion components relatively easily by lowering the plasma power or increasing the pressure; however, this solution would, at the same time, also reduce radical components, which are a raw material of carbon nanotubes. As another example, according to a commonly-used remote plasma method by which a substrate is positioned sufficiently distant from plasma, the quantity of radicals decreases like in the example where the pressure is increased. In addition, in the case where the life span of the ions is longer than the life span of the radicals, it becomes impossible to generate carbon nanotubes because the quantity of radicals being supplied is too small.
As a method for actively eliminating only the ion components, it has been proposed that, in combination with a remote plasma method, a mesh grid to which a bias can be applied is provided between plasma and a substrate so as to prevent the ion components from reaching the substrate. Because ions are positively charged normally, it is possible to eliminate only the ion components easily by applying a positive bias. In this situation, however, electrons that have the opposite electric charges (or negative ions may have been generated in some cases, depending on the type of gas being used) are accelerated by the positive bias and go through the mesh grid. In many situations, the bias that is applied for the purpose of eliminating the ion components is at a level of tens of volts or higher. Thus, the electrons that have gone through the mesh grid have a kinetic energy of tens of electron volts or higher. Because this kinetic energy is high enough to decompose and excite the gas, ions and radicals are re-generated between the mesh grid and the substrate. For this reason, in an experiment using a mesh grid to which a positive bias was applied, it was possible to obtain only low-quality carbon nanotubes in which the graphene wall was shaped in a “cup stack” form. In other words, by simply configuring the apparatus so that a positive bias is applied to the mesh grid, it is difficult to manufacture high-quality carbon nanotubes, although it is possible to eliminate the ion components.
Further, a method for eliminating the ion components by using a magnetic field has also been proposed. However, according to this method, substances that have the opposite electric charges are collected, like in the example of the electric method where the bias is applied. In addition, according to this method where a magnetic field is used, energy is given to both the ions and the electrons. Thus, it is more difficult to completely eliminate the ion components, because the gas is decomposed and excited by those ions and electrons in a more enhanced manner.
As explained above, the conventional techniques focus only on elimination of the ion components and take no countermeasures for particles having the opposite electric charges. As it is apparent from the result of the experiment described above, to have high-quality carbon nanotubes grow at a low temperature, it is important to address the issue of the particles having the opposite electric charges as well as to eliminate the ion components. Further, it is necessary to ensure that the radicals, which are required for the growth of the carbon nanotubes, will not be lost while the issue is addressed. Consequently, a plasma apparatus that is capable of supplying only radical components is in demand.